(1) Field of the Invention
The present invention generally relates to a magnetic disk apparatus, and more particularly to a magnetic disk apparatus which is built by combining a 3.5-inch disk drive unit and a 5-inch disk drive unit into a single unit that is capable of accessing both a 3.5-inch disk and a 5-inch disk.
(2) Description of the Prior Art
A magnetic disk device is built by combining a 3.5-inch disk drive unit and a 5-inch disk drive unit into a single unit which can access both a 3.5-inch disk and a 5-inch disk.
FIG. 1A shows a conventional magnetic disk device 10 in which a 3.5-inch disk drive unit 14 and a 5-inch disk drive unit 11 are combined. The 5-inch disk drive unit 11 includes a top plate 13 with a certain thickness. The top plate 13 has a mounting surface on which the 3.5-inch disk drive unit 14 is mounted by fixing units 12. A front panel 15 is arranged to cover the front face of the 5-inch disk drive unit 11 and the front face of the 3.5-inch disk drive unit 14. The front panel 15 has at its lower portion an insertion opening through which a 5-inch disk cartridge can be inserted, and has at its upper portion an insertion opening through which a 3.5-inch disk cartridge can be inserted.
It is necessary that the conventional magnetic disk device 10 has a given height "h", and the magnetic disk device can be housed in a personal computer. The top plate 13 of the 5-inch disk drive unit 11 must be rigid enough to safely support the 3.5-inch disk drive unit 14 thereon. Thus, it is necessary that the top plate 13 has a certain thickness t1, and the thickness of the top plate 13 is approximately 1 mm.
In the disk drive device 10 mentioned above, the 3.5-inch disk drive unit 14 and the 5-inch disk drive unit 11 must be assembled independently from each other.
FIG. 1B shows a disk motor 17 of the 3.5-inch disk drive unit 14 in the magnetic disk device 10. In FIG. 1B, the 3.5-inch disk drive unit 14 includes the disk motor 17, a chassis 20, a slider 21, a holder 22, and a cover 23. The disk motor 17 is arranged on a motor base 24 which is connected to an external interface circuit via a connecting cord. The chassis 20 has a recessed portion 20a at the bottom of the 3.5-inch disk drive unit 14. The motor base 24 is placed into the recessed portion 20a of the chassis 20. The motor base 24 does not project from the bottom of the chassis 20, and all the parts are placed so as to fall within the height of the 3.5-inch disk drive unit 14.
The disk motor 17 of the 3.5-inch disk drive unit 14 comprises a rotating shaft 30 rotatably supported by a set of bearings 31 and 32, a ring-like permanent magnet 36 fixed to the inside wall of a flat, cup-like yoke 33, and a stator coil 35 having an iron core 34 arranged on the motor base 24.
As the motor base 24 is placed into the recessed portion 20a of the chassis 20, the height (indicated by "a" in FIG. 1B) of the disk motor 17 becomes a small dimension. For this reason, the space in which the stator coil 35 and the permanent magnet 36 are placed is narrow. Further, the number of turns of wires of the stator coil 35 is limited, and the height (indicated by "b" in FIG. 1B) of the permanent magnet 36 must not exceed 3.8 mm.
There is a need for a magnetic disk device which is compact and low in the manufacturing cost. Further, in order to obtain a disk motor having the required magnetic characteristics while satisfying the need for the magnetic disk device which is compact and low in the manufacturing cost, it is necessary that the conventional magnetic disk device 10 uses a permanent magnet with an increased concentration of a certain component element in the permanent magnet. However, providing the permanent magnet having the increased concentration of the above component element makes the manufacturing cost of the magnetic disk device high.
In addition, when the magnetic disk device mentioned above is assembled, adjusting operations for the 3.5-inch disk drive unit must be performed twice; after the assembly of the 3.5-inch disk drive unit is completed and also after the 3.5-inch disk drive unit is attached to the 5-inch disk drive unit. Such adjusting operations are time consuming, and the manufacturing cost of the magnetic disk device is unnecessarily raised due to these operations.